This invention relates to flow regulating devices for oil and gas wells, and, more particularly, to adjustable chokes for metering of well fluid.
Adjustable chokes are commonly used in oil and gas wells to control the flow of a well fluid. Adjustable chokes not only provide flow and non-flow control, but they also enable the flow of a well fluid to be set to a specified rate.
The most commonly used adjustable chokes are those which utilize a needle valve to increase or decrease the area available for flow proximate an outlet port of the choke. Chokes of this type have a stem which tapers to a point at its end to form a needle. The stem is moved relative to a seat to vary the flow area.
The performance of the adjustable chokes heretofore available has been questionable, in all but applications involving relatively low flow rates. Prior art adjustable chokes have suffered from large pressure drops through the devices, causing a loss of needed pressure further downstream. Performance has been even further diminished in well applications where quantities of sand or other abrasive materials are entrained in the well fluid. This reduction in performance has resulted from the rapid wearing away and a disfigurement of choke components, such as the valve seat, by the abrasive materials. When this occurs, the flow of well fluid becomes even more turbulent. As turbulence increases, the increased pressure drop is added to the inherent pressure drop of the needle itself. Even the common flow bean may be severely abraded in such applications, increasing the turbulence present within it and causing an increased pressure drop.
It is apparent that an efficient, rugged, adjustable choke is highly desirable to permit longer intervals between inspections or replacements and to maintain a higher fluid pressure downstream from the device.
Abrasion problems also have been encountered in the past in downhole safety devices which are used to shut-in wells to terminate the flow of well fluid when a predetermined flow rate is exceeded. It has been found that a tapered plug and seat arrangement can be used to reduce deterioration of downhole safety valve components, resulting in greatly increased reliability. Such a device is shown in U.S. Pat. No. 3,757,816.
It will be observed that downhole safety valves and adjustable chokes are totally dissimilar devices. Neither the basic designs nor the objectives of the devices are the same. Safety valves are adapted for shutting in a well in response to a predetermined maximum flow condition, whereas adjustable chokes are adapted for metering the production flow of a well fluid. Further, the locations of utilization are different, downhole safety valves being used below the surface within the well bore, and adjustable chokes being used above ground.